1. Field of the Invention
The invention is related to a variable valve timing control apparatus for an internal combustion engine having a camshaft and a crankshaft, and the variable valve timing control apparatus is provided with an intermediate lock mechanism. The intermediate lock mechanism locks a VCT phase that is a rotational phase of the camshaft relative to the crankshaft at an intermediate lock position located within an adjustable range of the VCT phase.
2. Description of Related Art
In a conventional hydraulic variable valve timing unit, as described in JP-A-H9-324613 corresponding to U.S. Pat. No. 5,738,056 and JP-A-2001-159330 corresponding to U.S. Pat. No. 6,330,870, a lock position during the engine stop is designed to be located at a middle of an adjustable range of a variable cam timing (VCT) phase such that the adjustable range of the valve timing (VCT phase) is enlarged. In the above, the intermediate lock position, at which the VCT phase is locked during the stop of the engine, is set at a position suitable for the engine start. Thus, the engine is started while the VCT phase is at the intermediate lock position, and when the engine rotation (oil pump rotation) has increased after the starting of the engine is completed, and thereby the oil pressure has reached an appropriate oil pressure, the lock is released such that the VCT phase is controlled to a target VCT phase that is determined based on the engine operational state.
A hydraulic control valve controls oil pressure that actuates the variable valve timing unit. The hydraulic control valve may be abnormally operates when foreign objects are clogged in the valve element. For example, the foreign objects may be impurities, in oil that circulates in the hydraulic control valve. As a result, for example, the following abnormalities may occur, a lock abnormality, in which the VCT phase is not locked as required, a lock release abnormality, in which the lock of the VCT phase is not released as required, a VCT phase control abnormality, in which the VCT phase is not controlled to the target phase as required.
For example, as described in JP-A-H9-195805 and JP-A-2001-234768, upon the generation of a foreign-object-release request (cleaning request), a control amount (control duty) of the hydraulic control valve is alternately changed at predetermined intervals of time between a minimum value (0%) and a maximum value (100%) such that the clogged foreign objects are removed.
However, in a certain configuration of the hydraulic control valve, when the control amount (control duty) of the hydraulic control valve is alternately changed at predetermined periods of time between a minimum value (0%) and a maximum value (100%) upon the generation of the foreign-object-release request, the lock pin may periodically project and retract, and thereby the lock pin repeatedly may collide with a wall surface that defines the lock hole. As a result, abnormal noise may be generated, and the components may deteriorate disadvantageously.